1) Field of the Invention
The present invention relates to polarized light sources. More specifically this invention relates to polarized light sources with pre-selected bandwidths and several methods for making the same.
2) Background Information
The use of polarized light has become ubiquitous in modem technology, functioning as both a direct improvement for some applications and an enabling technology for others. For example, polarized indoor lighting may dramatically reduce glare from horizontal surfaces, reducing eyestrain and lowering power requirements by eliminating the unnecessary polarization components. Further, most liquid crystal display (LCD) panels used in wristwatches, notebook computers, and automobile dashboard displays require polarized light. Polarized light is usually obtained with the use of polarizing sheets or films that absorb over 50% of the incident light. Therefore, a device that contains an LCD operates at considerably less than optimal efficiency, wasting energy and therefore shortening battery life.
A light source that produces substantially pure polarized light would be a significant improvement over the use of polarizing sheets and films. Such a device may deliver a dramatic improvement to LCD and interior lighting performance, as well as play a key role in reducing glare in numerous situations. Reducing glare is important to reducing energy consumption and improving consumer safety.
The simplest way to produce polarized light is to place a polarizer over a conventional, unpolarized light source, thereby extinguishing one of the two polarization components. As described above, conventional polarizers absorb at least 50% of the incident light in order to transmit the desired polarization component, and therefore generally cannot be used to produce bright, efficient polarized light sources. One improved technology is to utilize polarization converters based on integrated polarizing beam splitters as described by Faris, in U.S. Pat. No. 5,096,520, and Kelly, in U.S. Pat. No. 5,394,253. However, such polarizing beam splitter sheets may be relatively bulky and heavy, and tend to be difficult to implement in applications requiring flat configurations. An alternative approach is the use of multiple polymer dielectric-layer (MPDL) based polarizers. These are described by Benson (in U.S. Pat. No. 5,831,375), Weber, et al., (in Science, vol. 287, p. 2451 (2000)), and Wortman, et al., (in U.S. Pat. No. 6,101,032). These Benson and Wortman patents are fully incorporated by reference herein. The MPDL based polarizers are constructed of multiple birefringence layers that are designed to reflect the desired polarization component and transmit the other. They, therefore, absorb very little light. Both Benson and Wortman, et al., disclose the use of MPDL based polarizers to construct polarized light sources. However, it is expected that difficulties would be encountered in manufacturing because precise control of the thickness and birefringence value is required for each layer. The manufacturing difficulties would be exacerbated for a broadband polarizer because thickness variation would be required in the different layers.
Recently, a number of researchers have reported the emission of polarized light from organic electroluminescent (OEL) and/or photoluminescent (PL) devices. For example, see Dyreklev, et al., in Adv. Mater., vol. 7, p. 43 (1995), Era, et al., in Appl. Phys. Lett., vol. 67, p. 2436 (1995), Cimrova, et al., in Adv. Mater., vol. 8, p. 146 (1996), Sariciftci, et al., in Adv. Mater., vol. 8, p. 651 (1996), and Montali, et al., in Nature, vol. 392, p. 261 (1998). These researchers concentrated on aligning organic molecules in a configuration such that polarized light emission was achieved. For example, Era et al., used epitaxial growth of organic materials on ordered films or rubbed films to obtain molecular alignment of the emitting materials, resulting in emission of linearly polarized light. However, as is typical for these methods, highly specific and costly materials, and a complicated vacuum deposition process are required. Manufacturing of multiple devices or of large-area devices would be prohibitively expensive and impractical using these methods.
A further method has been disclosed, whereby xcfx80-conjugated polymers are utilized to obtain emission of circularly polarized light (Peeters, et al., J. Am. Chem. Soc., vol. 119, p. 9909 (1997)). However, the polarization purity achieved was poor, resulting in only a fraction of a percent more light of one handedness than the other. Polarization purity was also the primary drawback of a recent grating-based technique reported by Suganuma in Appl. Phys. Lett., vol. 74, p. 1206 (1999).
The most promising processes reported to date rely on the self-assembling nature of cholesteric liquid crystal (CLC) molecules. Lussem, et al., in Liq. Cryst., vol. 21, p. 903 (1996), developed a light-emitting liquid crystalline polymer that was spin-coated on a rubbed polyimide surface to achieve alignment of the liquid crystal molecules. The light emissions were linearly polarized, however, the light-emitting polymer material is quite rare and expensive, thereby limiting the practicality of this method. Chen, et al., in Nature, vol, 397, p. 506 (1999), reported polarized light emission from a photoluminescent device using glassy chiral-nematic liquid crystal films. They observed strongly circularly polarized light emission in the reflective bands of these liquid crystals within the 400-420 nm wavelength range, and crossover between intensities of right-handed (RH) and left-handed (LH) circularly polarized light within these bands. These circularly polarized light devices have two serious disadvantages, however. First, since the polarized light emission only appears in the reflective band (as expected for a CLC polarizing device), broadband polarized light emission cannot be achieved. Second, the crossover behavior between RH and LH polarized light renders the materials generally unsuitable for display applications.
In conclusion, there are currently no low-cost methods for producing an efficient polarized light source that provides high purity polarized light.
The present invention is a novel polarized light source. The polarized light source of this invention includes an organic electroluminescent (OEL) device or an organic photoluminescent (OPL) device and a non-absorbing cholesteric liquid crystal (CLC) polarizing layer.
The polarized light source of the present invention is a highly efficient, high purity, and bright source of polarized light. The theoretical maximum light efficiency for this invention is 100%. Further, the present invention enables a polarized light source to be custom designed with a polarization bandwidth and position across a wide range of wavelengths. Further still, the light source of this invention is made from low cost materials and is easily manufactured. Yet further still, this invention enables the production of ultra-thin and lightweight polarized light sources.
In one embodiment, the present invention is a polarized light source comprising (i) an OEL device that includes a anode, an OEL material, and an indium tin oxide glass substrate anode or an OPL device that includes a mirror and an OPL material, (ii) a CLC polarizing device, and (iii) one or more glass substrates. This embodiment produces substantially pure circularly polarized light in the reflection bandwidth of the CLC polarizing device and unpolarized light at other wavelengths.
In another embodiment, the present invention is a polarized light source comprising (i) an OEL or OPL device as described in the previous embodiment, (ii) a CLC polarizing device, and (iii) a micro cavity capable of generating micro cavity resonance. The polarized light source of this embodiment may further comprise a birefringent retarder layer, positioned within the micro cavity. The birefringence value or the thickness of the birefringent retarder layer may be chosen such that the micro cavity generates micro cavity resonance at any desired wavelength.
In yet another embodiment, the present invention is a polarized light source comprising (i) an OEL device including a cathode, an indium tin oxide anode and a mixture of a cholesteric liquid crystal material and an organic electroluminescent material, wherein the mixture is sandwiched between the cathode and the indium tin oxide anode layer or an OPL device including a mirror and a mixture of a cholesteric liquid crystal material and an organic photoluminescent material, and (ii) one or more glass substrates. The CLC materials in the composite may have a constant pitch, thereby producing narrowband polarized light, or a pitch distribution, thereby producing broadband polarized light. The polarized light source of this embodiment may further comprise a CLC polarizing layer at the output side of the device to improve the purity of the polarized light.